1. Field of the Invention
This invention relates to a method for manufacturing ferrite powder used in ferrite cores, ferrite magnets, electromagnetic wave absorption members, magnetic recording media, and similar, and in particular relates to a method for manufacturing ferrite powder with fine particles.
2. Related Background Art
Liquid-phase reaction methods and vapor-phase methods are known as methods of manufacture of ferrite powder in which fine particles are obtained.
In general solid-phase reaction methods, first compounds serving as starting materials such as oxides, hydrides, carbides, and similar are mixed so as to obtain a prescribed composition. This mixture is placed in a ceramic or metal tray, and is heated to a prescribed temperature at a slow rate, such as for example 5° C./minute or less, in an air atmosphere or in a gas atmosphere at atmospheric pressure. Then, by performing heat treatment in which the material is held for 2 to 8 hours at 1000 to 1300° C., the starting materials are caused to react with each other (ferrite synthesis reaction), to obtain the desired ferrite powder. The ferrite powder thus obtained is crushed as necessary. Further, by performing heat treatment any strain occurring due to crushing can be removed (see for example Japanese Patent Laid-open No. S62-17841).
Solid-phase reactions entail heat treatment performed for approximately 2 to 8 hours, so that grain grown occurs simultaneously with the ferrite synthesis reaction. Hence the ferrite powder obtained by solid-phase reactions has comparatively large grain diameters, typically of micron order.
In a vapor-phase method, first organometallic complexes and other starting materials are heated, and converted into a gas or nearly-gaseous state. Then, the heated starting materials are transported to a reactor using a carrier gas or other means. And, the starting materials are decomposed by heat, or using laser light, ultraviolet rays, plasma, or other energy, to synthesize ferrite powder. Extremely fine powder can be obtained from vapor-phase methods, but the granularity distribution is broad, and the amounts synthesized are small, so that such methods are applied when producing expensive materials, and are not generally used as methods to produce fine and comparatively inexpensive powders such as ferrite.
Ferrite powder obtained by liquid-phase reaction methods has small particle sizes compared with ferrite powder obtained from solid-phase reaction methods. Coprecipitation methods and organic salt methods are well-known as liquid-phase methods.
In a coprecipitation method, first, a solution of a metal salt containing the metal to form the ferrite, and sodium hydroxide, potassium hydroxide, or another alkali metal hydroxide, are made to react chemically, to prepare a precipitate (precursor) comprising an oxide or hydroxide. This precipitate can be subjected to heat treatment (ferrite synthesis reaction), to synthesize fine-particle ferrite powder (see for example Japanese Patent Laid-open No. S61-29888). However, although fine particles can be obtained as a precursor by the coprecipitation method, the ferrite synthesis reaction is accompanied by grain growth. Hence it has been difficult to obtain ferrite powder with primary particles of size 200 nm or less, for example.
In an organic salt method, first citric acid, oxalic acid, or another organic acid is added to an aqueous solution of a metal salt containing the metal forming the ferrite, the solution is heated, a reaction is driven in the liquid phase, and an organic acid complex (precursor) is prepared. By subjecting this complex to heat treatment (ferrite synthesis reaction), a fine-particle ferrite powder can be synthesized. However, similarly to the coprecipitation method, during the heat treatment to drive the ferrite synthesis reaction, unnecessary grain growth occurs.
In order to perform heat treatment and drive the ferrite synthesis reaction without inducing grain growth of the fine precursor obtained by a liquid-phase reaction method, the precursor must be heated rapidly.
As a methods realize this rapid heating, high-frequency inductive heating using a heating coil, and an infrared heating furnace (Japanese Patent Laid-open No. 2001-284112) are known. However, these methods require a container to hold the starting material composition (processed material). Hence there are concerns of problems arising from the occurrence of difference phases due to reactions of processed material with the container, and from declines in cooling speed due to the thermal capacity of the container and of resulting grain growth. Further, because in these methods the processed material is placed at rest and heat treatment performed, the amount processed in one cycle is limited, so that there is the drawback that such methods are not suited to mass production.
Another method to realize rapid heating is disclosed in Japanese Patent Laid-open No. 2003-139469. The method of Japanese Patent Laid-open No. 2003-139469 is a heat treatment method in which the processed material is heated to a prescribed temperature, and after holding the processed material at that temperature for a prescribed time, is cooled. In this heat treatment method, while subjecting the processed material to free fall in vacuum, by heating the processed material during free fall from the surroundings, the processed material is heated to the prescribed temperature.
However, although in the method of Japanese Patent Laid-open No. 2003-139469 heating to a prescribed temperature is performed while causing free fall in vacuum, processing to maintain the prescribed temperature for the prescribed time is performed on a rotating drum (see FIG. 1 of Japanese Patent Laid-open No. 2003-139469). When, in such a method, ferrite powder is obtained from a precursor, the precursor, heated to the temperature at which the ferrite synthesis reaction occurs, falls onto and collides with the rotating drum. Hence the particles forming the precursor are compacted together. Further, because the material is held for a comparatively long time on the rotating drum, there are concerns that grain growth and strong agglomeration may occur. Further, in order to withstand sustained high temperatures, the rotating drum must be formed from a material which does not readily react with the processed material, and to which there is little adhesion. And, even when the rotating drum is formed from such material, considering processing over long periods of time and repetition of processing any number of times, it is undeniable that small amounts of processed material remain on the rotating drum. Hence it is easily inferred that the remaining processed material may become nuclei to promote grain growth and sintering of processed material.